The present invention relates to a non-invasive method and apparatus for detecting biological activities in a fluid specimen, such as blood. The specimen and a culture medium are introduced into a sealable container and exposed to conditions enabling metabolic processes to take place in the presence of microorganisms.
Usually, the presence of microorganisms such as bacteria in a patient's body fluids, particularly blood, is determined using blood culture vials. A small quantity of blood is injected through the sealing rubber septum into a sterile vial containing a culture medium. The vial is incubated at 37.degree. C. and monitored for bacterial growth.
Common visual inspection involves monitoring the turbidity of the liquid suspension. Known instrumental methods detect changes in the carbon dioxide content in the head space of the culture bottles, which is a metabolic by-product of the bacterial growth. Monitoring the carbon dioxide content can be accomplished by methods well established in the art, including radiochemical, infrared absorption at a carbon dioxide spectral line, or pressure/vacuum measurement. These methods, however, require invasive procedures which result in the well-known problem of cross-contamination. In case of vacuum/pressure measurement, on the other hand, any temperature change within the vial head space also generates a pressure change which is not related to biological activities. Therefore, an additional head space temperature measurement is required in order to distinguish between biological and temperature effects. Non-invasive head space temperature monitoring, however, represents an extremely difficult problem, and there are currently no practical solutions. Further, the metabolic activity of some microorganisms can result in very high head space pressures. This means that while a pressure sensor has to be sensitive in order to allow detection of diverse microorganism species, it must also be protected from extreme pressure. Depending on the technology used, these two requirements often contradict each other and cannot be simultaneously satisfied.
Recently, novel non-invasive methods have been developed which use chemical sensors inside a vial. Such sensors often respond to changes in the carbon dioxide concentration by changing their color or by changing their fluorescence intensity. The outputs from these sensors are based upon light intensity measurements. This means that errors may occur, particularly if the light sources used to excite the sensors, or the photodetectors used to monitor intensities, exhibit aging effects over time.
The disadvantages of intensity-based methods can be overcome by utilizing modulated excitation light in combination with fluorescent sensors that change their decay time in response to changing carbon dioxide concentration. Using this method, intensity measurements are replaced with time measurements, so intensity changes do not influence the results. Current fluorescent decay time sensors, however, require high brightness short-wavelength light sources (550 nm or shorter) that are intensity-modulated at very high frequencies (typically above 100 MHz). A preferred embodiment would be a 5-mW green helium neon (HeNe) laser (543.5 nm), externally modulated by means of an acousto-optic light modulator. The laser/modulator combination is expensive, and it is expected that in practice the vials would have to be moved to the laser instead of having a light source for each vial. Further, such instruments would have moving parts and the time interval between successive measurements for each vial would be relatively long. And, it is not likely that inexpensive high-brightness short-wavelength semiconductor diode lasers will be developed in the near future.